Proximity effect in normal metal/ferromagnet/d-wave superconductor structures

The superconducting proximity effect in normal metal/insulator/ferromagnet/d-wave superconductor (N/I/F/D) structures is studied based on an extended Blonder–Tinkham–Klapwijk (BTK) theory. The transition from the “0” to “π” state is found in the conductance spectra with increasing thickness of F or the ferromagnetic exchange energy. The superconducting proximity effect is drastically changed by the orientation angle α, as α increases the proximity effect is enhanced, being strongest for α/π = 0.25.     

Conductances in normal and normal/superconductor structures

We study theoretically electronic transport through a normal-metal/superconductor (NS) interface and show that more than one conductance may be defined, depending on the pair of chemical potentials whose difference one chooses to relate linearly to the current. We argue that the situation is analogous to that found for purely normal transport, where different conductance formulae can be invoked. We revisit the problem of the ‘right’ conductance formula in a simple language, and analyze its extension to the case of mesoscopic superconductivity. The well-known result that the standard conductance of an NS interface becomes 2 (in units of 2 e² / h) in the transmissive limit is viewed here in a different light. We show that it is not directly related to the presence of Andreev reflection, but rather to a particular choice of chemical potentials. This value of 2 is measurable because only one single-contact resistance is involved in a typical experimental setup, in contrast with the purely normal case where two of them intervene. We introduce an alternative NS conductance that diverges in the transmissive limit due to the inability of Andreev reflection to generate a voltage drop. We illustrate numerically how different choices of chemical potential can yield widely differingI –V curves for a given NS interface.     

Tunable supercurrent in superconductor/normal metal/superconductor Josephson junctions

When two superconductors are connected by a weak link a supercurrent flows determined by the difference in the macroscopic quantum phases of the superconductors. Originally, this phenomenon was discovered by Josephson for the case of a weak link formed by a thin tunnel barrier. The supercurrent I is related to the phase difference ϕ through the Josephson current–phase relation, I = I_csin ϕ, with I_c, the critical current, depending on the properties of the weak link. A similar relation holds for weak links consisting of a normal metal, a semiconductor or a constriction . In all cases, the phase differenceϕ =  0 when no supercurrent flows through the junction, and ϕ increases monotonically with increasing supercurrent until the critical current is reached. Using nanolithography techniques we have succeeded in making and studying a Josephson junction with a normal metal weak link, in which we have direct access to the microscopic current-carrying states inside the link. We find that the fundamental Josephson relation can be changed fromI = I_csin ϕ toI = I_csin(ϕ + π), i.e. to a π -junction, by suitably controlling the energy distribution of the current-carrying states in the normal metal. This fundamental change in the way these Josephson junctions behave has potential implications for their use in superconducting electronics as well as (quantum) logic circuits based on superconductors.     

Conductance of d-wave superconductor/normal metal/d-wave superconductor junctions

We develop a theory of the conductance of superconductor/normal metal/superconductor junctions in the case where the superconducting order parameter has d-wave symmetry. At low temperature the conductance is proportional to the square root of the inelastic electron relaxation time in the bulk of the superconductor. As a result it turns out to be much larger than the conductance of the normal part of the junction.     

Thermal effect in phase-periodic electron transport in disordered mesoscopic normal metal/superconductor structures

We report measurements of the temperature dependence of the amplitude of phase-periodic conductance oscillations in disordered normal metal (Ag) structures, attached to a superconducting (Al) wire at two points. The amplitude of oscillations reaches its maximum at temperatureT ^*, when the Thouless energy is of the order ofk _B T. The results are in agreement with recent calculations by Nazarov and Stoof [Phys. Rev. Lett. 76 (1996) 823].     

Conductance in normal metal/insulator/ferromagnetic superconductor tunnel junctions

In this paper, the superconducting order parameter and the energy spectrum of the Bogoliubov excitations are obtained from the Bogoliubov-de Gennes equation for a ferromagnetic superconductor (FS). In the framework of the Blonder-Tinkham-Klapwijk model, we present the differential conductance of the normal metal/insulator/FS junctions. It is shown that the exchange energy h in the FS can lead to the Zeeman splitting of the conductance peaks and the energy difference between the two splitting peaks is equal to 2h. The observation of such Zeeman splitting in the conductance spectrum can be taken as evidence for the coexistence between superconductivity and ferromagnetism.     

正常金属/自旋三重态p波超导体结隧道谱的奇异性

利用Blonder，Tinkham和Klapwijk理论计算了正常金属/绝缘层/正常金属/自旋三重态的p波超导体结的隧道谱和平均电流.计算结果表明：在自旋三重态p波超导结的隧道谱中存在零偏压电导峰、零偏压电导凹陷和双凹陷结构，并有微分电导随偏压震荡的现象出现，在I-V曲线上出现电流台阶.这些结果在理论上支持Sr₂RuO₄的超导态是自旋三重态p波超导态. 关键词： 自旋三重态超导体 p波超导体 隧道谱     

Spin-dependent proximity effects in ferromagnetic semiconductor/superconductor structures

Proximity effects in normal metal/insulator/ferromagnetic semiconductor/superconductor (NM/I/FS/SC) and NM/I/SC/FS junctions are studied based on an extended Blonder-Tinkham-Klapwijk (BTK) theory. It is found that the magnitude of the proximity effects depends to a great extent on the mismatches of the effective mass and band between the FS and SC. For NM/I/FS/SC junction, the transition of the tunneling conductance from “0” to “π” state is determined by the mass, magnetic exchange energy in FS and the thickness of FS. For NM/I/SC/FS junctions, the conductance spectrum is spin-dependent, indicating a local coexistence of weak ferromagnetism and s-wave superconductivity.     

超导体/正常金属/半导体/正常金属/超导体结的研究

在约瑟夫逊效应的理论研究中,大体上有微观和宏观两种方法。采用的是宏观的方法。利用推广的Ja-cobson方法,讨论了超导体/正常金属/半导体/正常金属/超导体结的约瑟夫逊效应,并在小电流假设下,推导出了该结的电流密度与位相之间的关系。     

Tunneling transport in topological insulator normal metal/insulator/d-wave superconductor junctions

We investigate the tunneling conductance in a normal metal/insulator/d-wave superconductor (NM/I/d-wave SC) junction with a barrier of thickness d and with an arbitrary gate voltage V(0) applied across the barrier region, formed on the surface of a topological insulator, using the Dirac-Bogoliubov-de Gennes equation and Blonder-Tinkham-Klapwijk?(BTK) formalism. We find that the tunneling conductance as a function of both d and V(0) displays an oscillatory behavior whose amplitude decreases with increase of V(0). We also find that when the Andreev resonant condition is met, the tunneling conductance approaches a maximum value of 2G(0), independent of the gate voltage V(0).     

Coexistence of ferromagnetism and superconductivity in normal mental/superconductor/ferromagnet structures

<正>We extend the Blonder,Tinkham and Klapwijk theory to the study of the inverse proximity effects in the normal mental/superconductor/ferromagnet structures.In the superconducting film,there are the gapless superconductivity and the spin-dependent density of states both within and without the energy gap.It indicates an appearance of the inverse-proximity-effect-induced ferromagnetism and a coexistence of ferromagnetism and superconductivity near the interface.The influence of exchange energy in the ferromagnet and barrier strength at the superconductor/ferromagnet interface on the inverse proximity effects is discussed.     

Proximity effects in normal mental/spin-splitting material/superconductor junctions

We extend the Blonder, Tinkham and Klapwijk (BTK) theory to the study of the coexistence between ferromagnetism and s-wave superconductivity in ferromagnet/superconductor (F/S) structures. It is found that the ferromagnetism and s-wave superconductivity can coexist near the F/S interface, which is induced by proximity effects. On the F side, the density of states (DOS) exhibits some superconducting-like properties, and it displays a damped oscillation from `0' to `$\piE-mail： hlya7505@yahoo.com.cn/qk/85823A/200710/25696438.htmlproximity effects, `\pi' state, Rashba spin--orbit coupling7210, 74503/3/2007 12:00:00 AMWe extend the Blonder, Tinkham and Klapwijk (BTK) theory to the study of the coexistence between ferromagnetism and s-wave superconductivity in ferromagnet/superconductor (F/S) structures. It is found that the ferromagnetism and s-wave superconductivity can coexist near the F/S interface, which is induced by proximity effects. On the F side, the density of states (DOS) exhibits some superconducting-like properties, and it displays a damped oscillation from `0' to `$\pi$' states with increasing either the thickness of F film or the exchange energy. We also study the influences of the spin-polarized exchange splitting in the F and the spin-degeneracy by Rashba spin--orbit coupling (RSOC) in the two-dimensional electron gas (2DGE) on the proximity effects. It is shown that the case of Rashba spin-degeneracy is very different from that of the spin-polarized exchange splitting.http://cpb.iphy.ac.cn/CN/10.1088/1009-1963/16/10/042https://cpb.iphy.ac.cn/CN/article/downloadArticleFile.do?attachType=PDF&id=1088232007-10-08' states with increasing either the thickness of F film or the exchange energy. We also study the influences of the spin-polarized exchange splitting in the F and the spin-degeneracy by Rashba spin--orbit coupling (RSOC) in the two-dimensional electron gas (2DGE) on the proximity effects. It is shown that the case of Rashba spin-degeneracy is very different from that of the spin-polarized exchange splitting.     

Current and heat transfer through the interface between a two-band superconductor and a normal metal

Current and heat transfer through the interface between a two-band superconductor and a normal metal has been theoretically considered taking into account the interband scattering at the interface. Cases of various symmetries of the order parameter of the two-band superconductor such as s ₊₊ and s _± have been analyzed. The bolometric applications of the contacts of two-band superconductors with a normal metal have been discussed.     

Thermoelectric effects in superconducting proximity structures

Attaching a superconductor in good contact with a normal metal gives rise to a proximity effect where the superconducting correlations leak into the normal metal. An additional contact close to the first one makes it possible to carry a supercurrent through the metal. Forcing this supercurrent flow along with an additional quasiparticle current from one or many normal-metal reservoirs leads many interesting effects. The supercurrent can be used to tune the local energy distribution function of the electrons. This mechanism also leads to finite thermoelectric effects even in the presence of electron–hole symmetry. Here we review these effects and discuss to which extent the existing observations of thermoelectric effects in metallic samples can be explained through the use of the dirty limit quasiclassical theory. PACS  74.25.Fy; 73.23.-b; 74.45.+c; 74.40.+k     

Differential conductance of normal metal-insulator-d-wave superconductor junctions with interface roughness

Within the framework of Blonder-Tinkham-Klapwijk (BTK) model and taking into account the interface roughness, we calculate the differential conductance of normal metal-insulator-d-wave superconductor junctions. We find that the tunneling spectrum depends strongly on both the incident angle of electrons and the crystalline axis orientation of the superconductor, and exhibits the zero-bias anomaly under suitable arrangements. Interestingly, even in the absence of the insulating layer, the tunneling spectrum of normal metal-d-wave superconductor junctions differs significantly from that for normal metal-s-wave superconductor due to the interface roughness. Moreover, both the tunnel conductance peak at the energy gap and the zero-bias conductance peak are suppressed by the interface roughness. Our results can explain many experimental measurements on the tunneling spectra of high-T_c superconductors.     

Charge transport in the normal metal/diffusive ferromagnet/s-wave superconductor junctions

Charge transport in the normal metal/insulator/diffusive ferromagnet/insulator/s-wave superconductor (N/I/DF/I/S) junctions is studied for various situations solving the Usadel equation under the Nazarov's generalized boundary condition. Conductance of the junction is calculated by changing the magnitude of the resistance in DF, Thouless energy in DF, the exchange field in DF, the transparencies of the insulating barriers. We have found a new broad peak around zero voltage as well as zero bias conductance peak splitting and dip splitting.     

Electron transport across the interface of a normal metal and a two-band superconductor with interband pairing

A consistent scheme taking into account both intra- and interband pairings has been proposed for diagonalizing the Hamiltonian of a superconductor. The temperature dependence of the order parameter of the superconductor with interband pairing has been calculated. The electron transport across the interface of a normal metal and the two-band superconductor with interband pairing has been analyzed.     

The superconductor/ionic conductor interface

This paper deals with the charge transfer across the interface superconductor/ionic conductor — a relatively unknown field at present. Both n-type classical and p- and n-type high T_c superconductors are considered. Different types of solid and liquid electrolytes are used in experiments covering temperature ranges down to 10 and 88 K, respectively. Transient technique, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry represent the electrochemical methods in the study of Faradaic charge transfer processes occurring at that interface. First results with solid electrolytes show a current doubling around T_c. This effect is interpreted as a quantum-electrochemical kinetic effect caused by Cooper pairs as correlated charge carriers.     

Switching effects in superconductor/ferromagnet/superconductor graphene junctions

The Josephson effect in the superconductor/ferromagnet/superconductor （SFS） graphene Josephson junction is studied using the Dirac Bogoliubov-de Gennes （DBdG） formalism. It is shown that the SFS graphene junction drives 0–π transition with the increasing of p=h0L/vF？, which captures the effects of both the exchange field and the length of the junction; the spin-down current is dominant. The 0 state is stable for p 〈 pc （critical value pc ≈ 0.80） and the π state is stable for p 〉 pc, where the free energy minima are at φg=0 and φg=π, respectively. The coexistence of the 0 and π states appears in the vicinity of pc.     

Screening currents in a normal metal backed by a superconductor

The Meissner effect in a normal metal N in which superconductivity is induced by contact with a superconductor S, has been studied by an r.f. permeability method. It is found that, when N itself is a superconductor below some critical temperature T_cN, a magnetic field can be screened, up to temperatures significantly higher than T_cN.